Determination of the Heterotrophic Activity of Epilithic Microbial Populations

The heterotrophic activity of planktonic and the numerically dominant epilithic bacterial populations in mountain streams was measured using radioactive labelled 14C-glutamic acid. The activities of these populations were compared, using a specific activity index based on the maximum number of cells per litre or square centimetre, determined by epifluorescence direct counting of the samples. Dispersed and in situ sessile bacterial populations from colonized stone squares were as much as 26 and eight times, respectively, more active per cell than the free-floating planktonic populations. The increased activity per cell of the disturbed epilithic population suggests that there is greater substrate accessibility for dispersed cells and indicates that at low substrate concentrations the polysaccharide matrix encasing the in situ population may be limiting the substrate diffusion. The uptake of glutamic acid, measured by the heterotrophic potential technique, clearly demonstrated that the epilithic population of the flowing stream actively contributed to the removal of organic molecules from the surrounding waters.